Impulse detection circuit

ABSTRACT

An impulse detection circuit for use in triggering circuits such as those used for detecting drum beats within background noise and then triggering music synthesizers in response to the drum beat. Differentiators and peak-rectifiers and filters are used to detect impulse like inputs over a wide dynamic range in a noisy background. The input signal is rectified and differentiated and then inputted into a peak-rectifier and filter having a fast charging and a slow discharging time constant. The output of this peak-rectifier and filter is differentiated and inputted into a pair-rectifier and filters operating in parallel, which filters have rapid charging and slow discharging time constants. The time constants of one of the two filters are approximately twice those of the other. The outputs of the two peak-rectifier and filters are compared in a comparator. Because of the differences in the time constants of the two filters, the comparator produces a pulse output in response to each impulse that is input to the circuit of the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to the electronic generation of music. Moreparticularly this invention pertains to "triggering" devices for usewith one or more electronic synthesizers. For instance "drum triggers"are used to control or "trigger" electronic synthesizers to producedrum-like sounds that have been stored in the digital memory of thesynthesizer. The input of the "drum trigger" is typically obtained froman electrical transducer attached to either a standard drum or drum pad.The live drummer strikes the drum or drum pad which causes the "drumtrigger," in turn, to output a pulse or trigger which causes theelectronic synthesizer to produce the desired drum sound.

2. Description of the Prior Art

Typically, a prior art "drum trigger" uses a peak amplitude thresholdcomparator for comparing the signal obtained from the drum or drum padto a reference threshold. The reference threshold is adjusted by thedrummer so that the "drum trigger" operates in response to the drummer'sstriking of the drum or drum pad with a drum stick. In such a simplepeak amplitude threshold comparator the threshold must be set at arelatively high level so that the drum trigger is not triggered bybackground sounds such as cross talk between drums or by the "ringing"of the drums. As a consequence this technique suffers from a lack ofsatisfactory operation over a wide dynamic range of drum strokes. Theprior art device also typically exhibits only a limited ability torespond accurately to rapid drumming patterns.

SUMMARY OF THE INVENTION

The present invention is a device which receives its input from thetransducer on the drum or drum pad and outputs a pulse to "trigger" theoutput of sound from the synthesizer. The present invention usesdifferentiators in combination with a peak-rectifier and filter and aparallel combination of a pair of peak-rectifier and filters withslightly different time constants in conjunction with a comparator toproduce an output pulse in response to a wide dynamic range of inputscorresponding to the signals received from the drum or drum pad.Although the primary application of the present invention is thetriggering of a synthesizer, it should be understood that the inventionmay be used in more general applications for the detection of impulsesin noisy signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a preferred embodiment of the invention.

FIG. 2 is a schematic diagram of a preferred embodiment of theinvention.

FIG. 3 depicts the form of a typical signal which appears at variouspoints in the circuit of the invention; and

FIG. 4 depicts typical outputs of the two peak-rectifier and filtercircuits that are inputted into the comparator.

FIG. 5 is a block digram of an embodiment of the invention using asingle differentiator.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1. The input signal to the invention is the signalthat is output from a transducer attached either to a drum or to a drumpad. This signal is input to the absolute value converter 1 depicted inFIG. 1. The absolute value converter 1 rectifies the input signal andthen outputs this rectified signal to the differentiator 2.Differentiator 2 then generates an output signal which is approximatelyproportional to the time rate of change of the rectified signal. Theoutput of differentiator 2 is then input to peak-rectifier and filter 3which, in turn, peak-rectifies and filters the signal applied to itsinput. The output of peak-rectifier and filter 3 is input to the inputsof differentiator 4 and differentiator 5 which, in turn, output signalsapproximately proportional to the time rate of change of the input tothese differentiators. The outputs from differentiators 4 and 5 areinput respectively to peak-rectifier and filters 6 and 7 whose outputsare compared in comparator 9 to produce a pre-selected voltage as theoutput from comparator 9 during the period that the voltage output frompeak-rectifier and filter 7 exceeds the voltage output by peak-rectifierand filter 6.

In the preferred embodiment the output from peak-rectifier and filter 6further includes a bias voltage provided by biaser 8. In manyapplications, the biaser 8 is not necessary for the detection ofimpulses in noise. However, when the invention is used as a drumtrigger, the addition of a bias from biaser 8 establishes a determinedstate for comparator 9 in the absence of an input signal so thattriggering pulses are not output during periods of no input signal. Theaddition of the bias also avoids "double-triggering" that otherwise mayoccur under some operational circumstances. It should be understood thatbiaser 8 could instead be connected to peak-rectifier and filter 7 ifthe sign of the bias were reversed.

Although the preferred embodiment includes differentiator 4 and 5, asdepicted in FIG. 5 a single differentiator could be used in their placewith the output of the single differentiator being connected to both ofthe inputs to peak-rectifier and filters 6 and 7. Absolute valueconverter 1, as well as differentiator 2 and peak-rectifier and filter 3also may also be omitted and the input signal from the drum transducerfed directly into the inputs of differentiator 4 and 5. However, theoperation of the invention is improved by the addition of theseelements.

The circuitry in the preferred embodiment for performing the functionsdepicted in FIG. 1 is shown in FIG. 2. In FIG. 2, the combination ofcircuit elements which together perform each of the functionaloperations depicted in FIG. 1 are enclosed in dashed line boxes that arenumbered in accord with the functions depicted in FIG. 1. For instance,in FIG. 2, the circuit elements depicted within dashed line box 1cooperate to produce an output which represents the absolute value orrectified value of the signal that is input to the invention. Theresistor and capacitor depicted within dashed line box 2 in FIG. 2operate as a differentiator so as to produce an output approximatelyequal to the time rate of change of the input to these elements. Thetime constant of the resistor-capacitor combination in dashed line box 2in the preferred embodiment is approximately 1 millisecond.

The transistor, resistor and capacitor depicted in dashed line box 3 ofFIG. 2 together operate as a peak-rectifier and filter that rapidlycharges the capacitor to the peak value of the input voltage to thetransistor and then discharges the capacity more slowly, all relative tothe steady state conditions for the capacitor. At steady state with nosignal input, the capacitor remains charged at a value nearly equal tothe collector voltage V_(cc). In reality the capacitor is discharged bythe input signal; however, in terms of dynamic changes, the capacitorcan be considered to be rapidly charged to the peak input value of theinput voltage and then discharged slowly thereafter. In the preferredembodiment the charging time constant is somewhat less than 1microsecond and the discharge time constant is approximately 15milliseconds. The output from peak-rectifier and filter 3 is input todifferentiators 4 and 5. The differentiation is provided by theresistors and capacitors depicted in dashed line box 4 and in dashedline box 5 in FIG. 2. The resistor-capacitor combinations have a timeconstant of approximately 4.4 milliseconds. The outputs fromdifferentiators 4 and 5 are input to the transistor, resistor andcapacitor combinations depicted in peak-rectifier and filter 6 and inpeak-rectifier and filter 7 of FIG. 2. The peak-rectifier and filter 6has a charging time constant of approximately 100 microseconds and adischarge time constant of approximately 50 milliseconds. Thepeak-rectifier and filter 7 has a shorter charging time constant ofapproximately 50 microseconds and a discharge time constant ofapproximately 25 milliseconds. As a consequence of the different timeconstants in peak-rectifier and filters 6 and 7, the output frompeak-rectifier and filter 6 is delayed in time relative to the outputfrom peak-rectifier and filter 7. These outputs and their temporalrelationship are depicted in FIG. 4.

The outputs of peak- rectifer and filters 6 and 7 are input tocomparator 9, which comparator produces a pre-selected output voltagerepresenting a logic "on" during the period of time that the output frompeak-rectifier and filter 7 exceeds the output from peak-rectifier andfilter 6. The output pulse may then be used to trigger the production ofa drum-like sound by the synthesizer. It is the output frompeak-rectifier and filter 6 which has a slower charging time and aslower (i.e. longer) discharge time than that of peak-rectifier filter 7that effectively operates as a dynamic, continually changing thresholdfor the detection of impulses in the input signal.

The resistor, diode and capacitor depicted in dashed line box 10 in FIG.2 provide biasing for the transistors utilized in the invention. FIG. 3depicts the typical signals as a function of time as they appear atvarious points in the circuitry of the invention. The points in thecircuitry at which the various signals appear are indicated by thelabels in FIG. 3.

I claim:
 1. A device for the detection of impulses in an electricalsignal comprising:a differentiator having an input and an output, thedifferentiator receiving the electrical signal at its input andgenerating at its output a differentiated signal approximating the timerate of change of the electrical signal, a first peak-rectifier andfilter having an input and an output, the first peak-rectifier andfilter receiving at its input the differentiated signal output by thedifferentiator, the first peak-rectifier and filter operating topeak-rectify and filter the signal at its input to generate at itsoutput a first peak-rectified and filtered signal, a secondpeak-rectifier and filter having an input and an output, the secondpeak-rectifier and filter also receiving at its input the differentiatedsignal output by the differentiator, the second peak-rectifier andfilter operating to peak-rectify and filter the signal at its input togenerate at its output a second peak-rectified and filtered signal, thefirst peak-rectifier and filter having a first set of filter timeconstants, and the second peak-rectifier and filter having a second setof filter time constants, the first set of filter time constantsdiffering from the second set of filter time constants, and a comparatorhaving a first input and a second input and an output, the comparatorreceiving at its first input the first peak-rectified and filteredsignal and receiving at its second input the second peak-rectified andfiltered signal, the comparator outputting a pre-selected signalwhenever the first peak-rectified and filtered signal is greater thanthe second peak-rectified and filtered signal.
 2. A device for thedetection of impulses in an electrical signal comprising:a firstdifferentiator having an input and an output, the first differentiatorreceiving the electrical signal at its input and generating at itsoutput a first differentiated signal approximating the time rate ofchange of the electrical signal, a first peak-rectifier and filterhaving an input and an output, the first peak-rectifier and filterreceiving at its input the first differentiated signal output by thefirst differentiator, the first-peak rectifier and filter operating topeak-rectify and filter the signal at its input to generate at itsoutput a first peak-rectified and filtered signal, a seconddifferentiator having an input and an output, the second differentiatoralso receiving the electrical signal at its input and generating at itsoutput a second differentiated signal approximating the time rate ofchange of the electrical signal, a second peak-rectifier and filterhaving an input and an output, the second-peak rectifier and filterreceiving at its input the second differentiated signal output by thesecond differentiator, the second-peak rectifier and filter operating topeak-rectify and filter the signal at its input to generate at itsoutput a second peak-rectified and filtered signal, the firstpeak-rectifier and filter having a first set of filter time constants,and the second peak-rectifier and filter having a second set of filtertime constants, the first set of filter time constants differing fromthe second set of filter time constants, and a comparator having a firstinput and a second input and an output, the comparator receiving at itsfirst input the first peak-rectified and filtered signal and receivingat its second input the second peak-rectified and filtered signal, thecomparator outputting a pre-selected signal whenever the firstpeak-rectified and filtered signal is greater than the secondpeak-rectified and filtered signal.
 3. The device described in claim 1and further comprising:a third differentiator having an input and anoutput, the third differentiator receiving the electrical signal at itsinput and generating at its output a third differentiated signalapproximating the time rate of change of the electrical signal, a thirdpeak-rectifier and filter having an input and an output, the thirdpeak-rectifier and filter receiving at its input the thirddifferentiated signal output by the third differentiator, the thirdpeak-rectifier and filter operating to peak-rectify and filter thesignal at its input to generate at its output a third peak-rectified andfiltered signal, the output of the third peak-rectifier and filter beingconnected to and providing the signals input to the differentiatorrecited in claim
 1. 4. The device described in claim 2 and furthercomprising:a third differentiator having an input and an output, thethird differentiator receiving the electrical signal at its input andgenerating at its output a third differentiated signal approximating thetime rate of change of the electrical signal, a third peak-rectifier andfilter having an input and an output, the third peak-rectifier andfilter receiving at its input the third differentiated signal output bythe third differentiator, the third peak-rectifier and filter operatingto peak-rectify and filter the signal at its input to generate at itsoutput a third peak-rectified and filtered signal, the output of thethird peak-rectifier and filter being connected to and providing thesignals input to the first and second differentiators recited in claim2.
 5. The device described in claim 3 and further comprising:an absolutevalue converter having an input and an output, the absolute valueconverter receiving the electrical signal at its input and generating atits output a signal approximating the absolute value of the electricalsignal, the output of the absolute value converter being connected tothe input of the third differentiator recited in claim
 3. 6. The devicedescribed in claim 4 and further comprising:an absolute value converterhaving an input and an output, the absolute value converter receivingthe electrical signal at its input and generating at its output a signalapproximating the absolute value of the electrical signal, the output ofthe absolute value converter being connected to the input of the thirddifferentiator recited in claim
 4. 7. The device described in any one ofclaims 1-6 and further comprising:a biaser connected to one of saidfirst or second peak-rectifier and filters, the biaser adding apre-selected bias to the signal output by said first or secondpeak-rectifier and filter.